JP Morgan's Ken Goldman analyzed Nielsen data on food prices and found that for every 12-week period since May 14, 2011, Walmart has been raising food prices faster than its competitors. Here's an excerpt from his note:

We recently looked at a basket of prices among commonly-bought food items, including bread, milk and coffee, provided by The Nielsen Company. We found that food prices are rising faster in Walmart than in most other channels. In the 12-week period ending 10/29/11, the basket sold in Food, Drug and Mass Merchandising channels including Walmart (FDM+) rose by 8.9% year-on-year. This same basket rose by only 8.6% Y/Y in these same channels excluding Walmart (FDMx). This is not a new trend; according to our data, in every 12-week period since 5/14/11, year-on- year FDM+ pricing has outpaced FDMx pricing.

Walmart's prices have been outpacing those of competitors in most major categories. See the chart below: